Ophthalmic lens is defined as a lens suitable for carrying on the eye or inside the eye. Also included are less common vision correction lenses such as artificial corneal and lamellar corneal implants.
A fixed single power lens provides good quality of vision but only within a small range of viewing distances that is usually significantly narrower than the range required from near to distant vision. There is a significant effort to develop a lens for presbyopia correction in a form of refractive or diffractive type lenses. This type of the implant provides a number of powers, so called bifocal or multifocal lens. Reference to bifocal or multifocal terminology is used herein interchangeably. The multifocal ophthalmic lens can provide refractive powers, diffractive powers or a combination of both. A multifocal diffractive lens includes multifocal diffractive surface to provide near focus and opposite refractive surface. The “opposite surface” means the refractive surface of the diffractive lens which is opposite to the surface with a diffractive zone with light passing through both the region of the opposite refractive surface and diffractive zone.
A diffractive lens generally consists of a number of annular surface zones of equal area, so called Fresnel type zones or grooves. The optical steps are provided between the adjacent zones that follow the specific rule hereinbelow described. If step sizes are zero or randomly sized or groove areas are randomly sized, the lens becomes a refractive type, i.e. the corresponding image locations are defined by Snell's law.
A diffractive lens can be considered as a combination of refractive surface formed by zero step size so called base curve or base surface and phase grating, see FIG. 1. Per the prior art, base surface is being spherical surface or aspheric surface that corrects for spherical aberration of the spherical refractive lens of the equivalent power. Equivalent power lens in general means that that original lens and the it is compared to as the lens of the equivalent power both have the same power surfaces as well. In this respect one can reference to equivalent power surface also.
A spherical refractive lens forms a quasi-spherical wavefront deviated from a perfect sphere due to a spherical aberration. The aspheric lens that corrects for spherical aberration creates a spherical wavefront. The amount of aberration produced by a spherical ophthalmic lens is usually small enough for pupils up to about 4 mm to assume the spherical shape of the corresponding wavefront. As a result, both spherical lens and aspheric refractive lens with spherical aberration correction are treated the same for phase grating calculation in the diffractive optic. From this stand point both types of diffractive lenses with spherical base surface and aspheric base surface with spherical aberration correction, are equivalent for phase (diffraction) grating calculation and, therefore, will be referenced in this disclosure as “spherical lens” or “spherical surface” to avoid a multiple repetition of the reference to “aspheric with correction for spherical aberration”.
A phase grating can be formed by different types of zone or groove shapes with a blaze shape shown on the FIG. 1 being the most common one. A specific periodic blaze shape is cut into a spherical refractive surface which becomes the base surface of the diffractive surface or diffractive lens. The resulted blaze shapes create a phase grating, i.e. a periodic array of optical scattering regions. This disclosure will use blaze grating as an example but the present invention is applied to any type of phase grating that produces distance and near foci.
A periodic structure of the phase grating is such that it creates constructive interference of light at different angles depending on wavelength of light which are called diffraction orders. The corresponding wavelength of light used to design the phase grating is called design wavelength. The directions of the diffraction orders and corresponding image locations are defined by the Grating formula, not Snell's law. The key point for the phase grating to form distinct diffraction orders, is to have equal areas of the grooves and equal Optical Path Differences (OPD) between adjacent grooves at their borders in the direction of each diffraction order.
The distances from the grating to the foci created by the diffraction orders can be quantified in terms of diffraction powers associated with the diffraction orders similarly to a refractive lens power definition. Zero-order diffractive power of the diffractive lens coincides with the refractive power of the refractive lens formed by the base surface of the diffractive lens and its opposite refractive surface and, as a result, loosely called refractive power of the diffractive lens.
According to the wave nature of light, constructive interference of light occurs if electromagnetic wave of light is in phase at the corresponding image location. The constructive interference is maintained if the light from the grooves is shifted by the full phase equaled to integer number of the design wavelength. For instance, zero order corresponds to the original direction of the light produced by the refractive lens formed by the base surface and opposite surface of the diffractive lens, i.e. zero phase shift between light coming from each adjacent blaze zone; 1st order is produced by the phase of one wavelength shift between each adjacent blaze; 2nd order is produced by the phase of two wavelengths shift between each adjacent blaze and so on. Thus, grating period or groove width determines an angle of the given diffractive order from the zero-order direction, i.e. the location of a higher order focus from the zero-order focus and, therefore, the responsible for the corresponding diffractive powers of high diffraction orders.
By the law of formation of a diffraction order, light can only be channeled along the diffraction orders of the diffractive lens where constructive interference can take place. It leads to the discrete foci of a diffractive lens. In addition, the image is physically formed at a given foci if a measurable percent of total light is actually channeled along a given diffraction order. This depends upon the light phase shift introduced by each blaze zone, i.e. blaze material thickness (h), FIG. 1. The percent of total light at a given order is called diffraction efficiency of this order. In general, one can call it also a light transmittance for the given order.
According to the “geometrical model” of the grating, 100% efficiency (light transmittance) in m-order can be achieved if the direction of the blaze ray defined by the refraction at the blaze coincides with the direction of m-order diffraction, (Carmiña Londoño and Peter P. Clack, Modeling diffraction efficiency effects when designing hybrid diffractive lens systems, Appl. Opt. 31, 2248-2252 (1992)). It simply means that the blaze material thickness is designed to direct the blaze ray along the m-order diffraction produced by the blaze groove widths for the design wavelength of light.
The “geometrical model” provides a simple explanation of the diffractive lens structure which is important in explaining the present invention instead of introducing the mathematics of phase function, transmission function and its Fourier series to calculate diffraction efficiencies and solving the diffraction integral for light intensity distribution between the diffraction orders.
For instance, if the blaze ray is refracted along the middle direction between zero-order and (−1)-order, then the diffraction efficiency is equally split between zero-order and (−1)-order. The corresponding blaze height is half of the one required for 100% efficiency for (−1)-order to allow the groove surface to refract the blaze ray in the middle direction. Still one has to go through a formal process of calculation to determine that the diffraction efficiencies of (−1)-order and zero-order each equals to 40.5% at the design wavelength with the rest of light is spread out between other orders of diffraction. The described above diffraction lens is a typical configuration of a bifocal (multifocal) diffractive lens used for contact lens and intra-ocular lens platforms.
In a simple paraxial form the circular grating zones, also called grooves, echelettes or surface-relief profile, can be expressed by the formula rj2=jmλf, i.e. the focal length of m-order diffraction (m=±1, ±2, etc) for the design wavelength (λ) can be closely approximated by the following formula:
      f    m    =            r      j      2              jm      ⁢                          ⁢      λ      
This is the formula used in the prior art of the groove widths calculation in multifocal diffractive optic with spherical base surface or aspheric base surface that corrects for spherical aberration of the equivalent power spherical lens, The location of groove's borders are simply determined by rj. The formula (1) is based upon the condition that the wavefront produced by the equivalent power refractive lens formed by the base surface of the diffractive lens and its opposite refractive surface is a spherical wavefront that focuses into a single point-focus at each diffraction order.
In the paraxial approximation the blaze material thickness to produce 100% efficiency at m-order is
                              h          m                =                              m            ⁢                                                  ⁢            λ                                (                          n              -                              n                ′                                      )                                              (        2        )            where n=refractive index of the lens material and n′=refractive index of the surrounding medium. Half of the blaze thickness in the formula (2) is used to produce bifocal diffractive lens with 40.5% of light directed to zero-order allocated to far focus and (−1)-order allocated to near focus, i.e. m=1.
A diffractive surface may be formed by different shapes of the periodic diffractive structure and not only by a blaze shape and for the generality of the present invention the term “groove” is used as the description of the variety of shapes of the diffractive structure including multiorder phase grating.
U.S. Pat. No. 5,096,285 by Silberman describes diffraction surface with 100% efficiency to provide single diffraction power and the invention does not utilize the main advantage of the diffractive optic to use several diffraction orders (zero and −1, or +1 and −1, etc.) to reduce pupil dependency of the bifocal ophthalmic lens performance.
U.S. Appl. No. 20050057720 by Morris describes also diffractive 100% efficiency surface with the utilization of multiorder diffractive surface (MOD), i.e. the zones having boundary condition of phase shift by the multiple wavelength to provide similar diffraction efficiency for the range of wavelengths instead of only for the design wavelength.
Cohen and Freeman are the principal inventors of ophthalmic multifocal diffractive optic that utilizes several diffractive orders to form image from the objects at different distances. The Cohen patents: U.S. Pat. Nos. 4,210,391; 4,338,005; 4,340,283; 4,881,805; 4,995,714; 4,995,715; 5,054,905; 5,056,908; 5,117,306; 5,120,120; 5,121,979; 5,121,980 and 5,144,483. The Freeman patents: U.S. Pat. Nos. 4,637,697; 4,641,934; 4,642,112; 4,655,565, 5,296,881 and 5,748,28 where the U.S. Pat. No. 4,637,697 references to the blaze as well as step-shapes (binary) diffractive surface.
Other patents on diffractive lenses have been granted to Futhey: U.S. Pat. Nos. 4,830,481, 4,936,666, 5,129,718 and 5,229,797; Taboury: U.S. Pat. No. 5,104,212; Isaacson: U.S. Pat. No. 5,152,788; Simpson: U.S. Pat. Nos. 5,076,684 and 5,116,111 and Fiola: U.S. Pat. Nos. 6,120,148 and 6,536,899.
Swanson in U.S. Pat. No. 5,344,447 describes tri-focal lens using binary type diffractive surface profile. Kosoburd in U.S. Pat. No. 5,760,871 also describes tri-focal lens with blaze and binary profiles.
Several patents describe the variable step size between the adjacent zones of the diffractive structure to control light transmittance at different diffraction orders with pupil size: U.S. Pat. Nos. 4,881,805 and 5,054,905 by Cohen describe so called progressive intensity bifocal lens where the step size at the adjacent zones reduced towards periphery to shift larger portion of light towards zero-order (far focus) diffraction image, i.e. to control light transmittance to the given order with pupil diameter. Baude et al in U.S. Pat. No. 5,114,220 discloses an ophthalmic lens which characteristically comprises at least two concentric regions having diffractive components with different phase profiles in order to use different orders of diffraction. Lee et al in U.S. Pat. No. 5,699,142 incorporates a similar concept into so called apodized lens by recommending the specific reduction in echelettes heights, so called apodization the diffractive surface echelettes heights, to split light initially equally between Far and Near foci (40.5% efficiency for each) and then the heights reduce towards lens periphery to shift larger portion of light towards far focus with larger pupil size, i.e. to control light transmittance with pupil diameter. Freeman in U.S. Pat. No. 5,748,282 also refers to the variable step size to control light intensity between different orders with pupil size variation.
U.S. Pat. No. 5,056,908 discloses an ophthalmic contact lens with a phase plate and a pure refractive portion within its optic zone that is placed at the periphery of phase zone area. U.S. Pat. No. 5,089,023 by Swanson also describes the lens with a combination of single focus refractive and diffractive segments that can be of bifocal design. In both inventions the refractive portion coincides with one of the diffractive order either for distant or near vision.
Tecnis multifocal diffractive lens (Tecnis MIOL) by Abbott Medical Optics includes refractive aspheric surface as the opposite surface to the diffractive surface. This aspheric surface is the front surface of the lens and multifocal diffractive structure is placed on the back of the lens. The lens formed by aspheric opposite and spherical base surfaces create distant focus (zero-order) and the diffractive structure produces near focus as (−1) order diffraction. The aspheric surface is to correct for spherical aberration of the equivalent power spherical lens and is to improve image contract at distant vision for large pupils above 4 mm diameter as compared with equivalent power bifocal diffractive lens with opposite spherical refractive surface.
U.S. Pat. Appl. No. 2006/0116764 by Simpson describes an aspheric multifocal diffractive lens with the base surface serving as an aspheric surface of the multifocal diffractive surface. The opposite refractive surface is spherical surface and together with aspheric base surface forms aspheric lens that is technically equivalent to Tecnis aspheric lens formed by aspheric opposite and spherical base surfaces as the aspheric base surface is also to correct for spherical aberration of the equivalent power bifocal diffractive lens with spherical base surface in order to improve image contrast of distant image at large pupils as compared with a multifocal diffractive lens with spherical base surface of equivalent power.
There is a downside in aspherization per Tecnis MIOL and Simpson's designs as it increases sensitivity to refractive error due to reduced Depth of Focus at far focus. A reduction in aberrations improves the vision quality (image contrast) as compared with spherical lens of equivalent power but reduces a depth of focus meaning more rapid image quality degradation with a small deviation from the best focus position. Best focus position is defined as the position where the image quality is the best as defined by a selected quality metric. For instance, smallest spot size, maximum contrast, maximum Modulation transfer function or maximum Strehl ratio.
It has been shown that bifocal diffractive lens demonstrates two distinct intensities at two foci for distant and near vision (Golub M A, et al, Computer generated diffractive multifocal lens. J. Modern Opt., 39, 1245-1251 (1992), Simpson M J. Diffractive multifocal intraocular lens image quality. Appl. Optics, 31, 3621-3626 (1992) and Fiala W and Pingitzer J. Analytical approach to diffractive multifocal lenses. Eur. Phys. J. AP 9, 227-234 (2000)). Diffractive optic offers the advantage to provide these foci independently to pupil diameter. Nevertheless, common to all diffractive designs of is the fact that a bifocal diffractive lens is lacking intermediate vision because there is no provision for intermediate focus.
All prior art multifocal diffractive lenses including Tecnis multifocal and the design described by Simpson in U.S. Pat. Appl. No. 2006/0116764 are lacking important attributes to further advance multifocal ophthalmic optic:                (a) intermediate focus (viewing of a computer screen, for instance) and        (b) low sensitivity to refraction error as the lens performance should not depredate significantly with inherent errors of IOL power calculation.        
In U.S. Pat. No. 7,073,906 by Portney intermediate focus was introduced to multifocal diffractive lens through refractive power by placing a refractive zone of multifocal power internally to a diffractive bifocal zone which produced only distant and near foci. The present invention expends the intermediate power region to the diffractive power. The regions is also to increase Depth of Focus at distance focus as compared with equivalent power diffractive lens with spherical opposite and base surfaces.
The objective of the present invention is to provide a multifocal diffractive lens that offers a vision range from far through intermediate to near. The related objective of the present invention is to provide multifocal diffractive lens with extended depth of focus (DOF) at far image in order to increase tolerance of distant vision to refraction error.